1. Field of the Invention
The present invention relates to a transmission power control method, a base station apparatus and a communication terminal and more particularly, is preferably applied to a cellular radio communication system.
2. Description of the Related Art
In a cellular radio communication system, an area in which communication service is provided is divided into cells of a desired size, a base stations is located in each of the cells as a fixed station, and a communication terminal as a mobile station radio-communicates with the base station whose communication condition is considered to be the best.
In such a cellular radio communication system, large transmission power is required for transmission or even small transmission power is enough for communication, depending on the location of the mobile station when desired communication is performed.
Accordingly, in the cellular radio communication system, the base station and the communication terminal monitor a reception power (or quality of the reception power) each other, and form a feedback loop by notifying the monitoring result to each other, to perform transmission power control for communication with the minimum transmission power.
Thus, the cellular radio communication system enables communication to be efficiently performed with the minimum transmission power, so that the power consumption can be reduced as compared with the case where the communication is performed with fixed power. Therefore, a communication terminal can be particularly provided with a distinctive advantage that a lifetime of a battery can be prolonged. Such a cellular radio communication system is described as follows.
In FIG. 1, 1 generally represents a cellular radio communication system of the time division multiple access (TDMA) method, which performs the communication by connecting a base station 2 to a communication terminal 3 with a radio circuit. In this case, the base station 2 comprises a reception unit 4, a control unit 5 and a transmission unit 6, and the communication terminal 3 also comprises a reception unit 7, a control unit 8 and a transmission unit 9. The base station 2 and the communication terminal 3 use these circuit blocks to communicate with each other.
The reception unit 4 of the base station 2 receives a transmission signal from the communication terminal 3, demodulates the incoming transmission data and detects the control data, included in the transmission signal, for power control, and transmits the detected control data to the control unit 5. Further, the reception unit 4 detects a signal-to-interference wave power ratio C/I (the ratio of the desired wave power and the interference wave power) of the transmission signal from the communication terminal 3, and also transmits the detected signal-to-interference wave power ratio C/I to the control unit 5.
The control unit 5 generates a power control signal for controlling the transmission power of the local station based on the control data from the reception unit 4, and sends it out to the transmission unit 6. Moreover, the control unit 5 generates control data for controlling the transmission power of the communication terminal 3 based on the signal-to-interference wave power ratio C/I from the reception unit 4, and also sends it out to the transmission unit 6.
The transmission unit 6 controls the transmission power of the local station based on the power control signal supplied from the control unit 5, and inserts the control data supplied from the control unit 5 into the transmission data in order to generate a transmission signal, and sends the transmission signal to the communication terminal 3.
In the same way, the reception unit 7 of the communication terminal 3 receives the transmission signal from the base station 2, demodulates the incoming transmission data and detects the control data, included in the transmission signal, for power control, and transmits the detected control data to the control unit 8. Further, the reception unit 7 detects a signal-to-interference wave power ratio C/I of the transmission signal from the base station 2, and transmits the detected signal-to-interference wave power ratio C/I to the control unit 8.
The control unit 8 generates a power control signal for controlling the transmission power of the local station based on the control data from the reception unit 7, and sends it out to the transmission unit 9. Moreover, the control unit 8 generates control data for controlling the transmission power of the base station 2 based on the signal-to-interference wave power ratio C/I from the reception unit 7, and also sends it out to the transmission unit 9.
The transmission unit 9 controls the transmission power of the local station based on the power control signal received from the control unit 8, and inserts the control data supplied from the control unit 8 into transmission data in order to generate a transmission signal, and sends the transmission signal to the base station 2.
In the transmission units 6 and 9 of the cellular radio communication system 1, frequency hopping (FH) is performed to randomly change the frequency channel to be used, according to a predetermined pattern for every slot, so that the effects of the interference waves from other communications can be reduced.
As described above, in the cellular radio communication system 1, the base station 2 and the communication terminal 3 detect the signal-to-interference wave power ratio C/I of the transmission signal from each other, and notify the control data of the transmission power according to the detection result to each other, thereby to control the transmission power.
With respect to the cellular radio communication system 1, the flowchart of FIG. 2 is used to explain the transmission power control procedure in the control unit 8, which controls the transmission power of the communication terminal 3 based on the control data for the power control supplied from the base station 2.
The control unit 8 of the communication terminal 3 begins with the starting step RT1, and moves to the step SP1. In the step SP1, the control unit 8 first accepts a power control command from the reception unit 7, and moves to the step SP2.
In the step SP2, the control unit 8 determines whether the power control command is a power-up command, which means increase of the transmission power, or not. If the positive result is obtained, the result means a power-up command is accepted, then the control unit 8 moves to the step SP3.
In the step SP3, the control unit 8 determines whether the current transmission power is smaller than the maximum transmission power or not. If the positive result is obtained, the result means there is room to further increase the transmission power, and the control unit 8 moves to the step SP4. On the other hand, if the negative result is obtained in the step SP3, the result means that the current transmission power has already reached the maximum transmission power and the transmission power cannot be increased any more, and the control unit 8 returns to the step SP1 again while maintaining this state.
In the step SP4, the control unit 8 sends out the power-up command to the transmission unit 9 to increase the transmission power by a predetermined level with the transmission unit 9, and returns to the step SP1 again. However, if the negative result is obtained in the step SP2, the result means a power-down command, not a power-up command, has been accepted, and the control unit 8 moves to the step SP5.
In the step SP5, the control unit 8 determines whether the current transmission power is larger than the minimum transmission power or not. The positive result is obtained, the result means there is room to further lower the transmission power, the control unit 8 moves to the step SP6. In the step SP6, the control unit 8 sends out the power-down command to the transmission unit 9 to lower the transmission power by a predetermined level with the transmission unit 9, and returns to the step SP1 again.
On the other hand, if the negative result is obtained in the step SP5, the result means that the current transmission power has already reached the minimum transmission power and the transmission power cannot be lowered any more, and the control unit 8 returns to the step SP1 again while maintaining this state.
In the cellular radio communication system 1 having such configuration, the base station 2 and the communication terminal 3 monitor the reception power (or quality of the reception power) each other, and notify the monitoring result to each other, to form a feedback loop for performing the transmission power control. However, a transmission error can occur during the transmission of the monitoring result to each other through the feedback loop, so that the power-up command and the power-down command are inverted respectively.
For instance, in the communication terminal 3, when a power-up command is repeatedly sent from the base station 2 regardless of the current transmission power being the maximum because the base station 2 and the communication terminal 3 are distant from each other, if a power-down command is sent due to a transmission error, the transmission power is lowered according to the power-down command even though the transmission power is desired to be increased because the transmission power is obviously insufficient. As a result, the cellular radio communication system 1 has a problem that communication quality cannot be maintained between the base station 2 and the communication terminal 3.
Further, in the communication terminal 3, when a power-down command is repeatedly sent from the base station 2 regardless of the current transmission power being the minimum because the distance between the base station 2 and the communication terminal 3 has become shorter, if a power-up command is sent due to a transmission error, the transmission power is increased according to the power-up command even though the transmission power is desired to be lowered because the transmission power is obviously sufficient. As a result, the communication with an optimum transmission power according to the distance between the base station 2 and the communication terminal 3 cannot be performed, thus the cellular radio communication system 1 has problems that an increase in the power consumption is caused and an interference wave to other channels occurs.
Further, in the cellular radio communication system 1 of the TDMA method, the frequency hopping (FH) is performed to randomly change the frequency channel to be used according to a predetermined pattern (diagonally shaded portions) for each slot, as shown in FIG. 3. In this case, even though the base station 2 and the communication terminal 3 are distant from each other and the current transmission power should be maximized, if a certain slot SLT1 accidentally becomes immune against the effect of the interference waves from other communications so that the communication condition becomes good, then a power-down command can be supplied from the base station 2.
In such case, if the communication terminal 3 lowers the transmission power according to the accidental power-down command, the transmission power is short again in the next frequency hopping. Accordingly, since it is possible that a power-down command due to some error is sent, the communication terminal 3 has a problem that communication quality cannot be maintained if the transmission power is easily lowered.